1. Field of the Invention
The present invention relates to a radiation detector, more specifically, to a radiation detector that employs a combination of scintillator and solid state photodetector.
2. Description of the Related Art
Radiation detectors formed of a solid state photodetector and a scintillator, which converts radiation to visible light, layered on top of another are proposed as described, for example, in Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, U.S. Pat. No. 5,187,396, and “Signal, noise, and readout considerations in the development of amorphous silicon photodiode arrays for radiotherapy and diagnostic x-ray imaging”, L. E. Antonuk et al., SPIE, Vol. 1443, pp. 108-119, 1991. The solid state photodetector includes, for example, a substrate made of silica glass with a thickness of 3 mm on which a transparent conductive layer and a conductive layer are formed with an amorphous semiconductor layer sandwiched therebetween. The transparent conductive layer includes a plurality of signal lines, and the conductive layer includes a plurality of scanning lines, which are pattern formed in a matrix form so that they are disposed orthogonal to each other.
In the radiation detectors described above, however, the solid state photodetector is disposed behind the scintillator viewed from the direction from which the radiation is irradiated, so that the visible light converted by the scintillator is absorbed or scattered by the scintillator itself before reaching the solid state photodetector. Consequently, the visible light detection efficiency of the photodetector is deteriorated, and the sharpness of the obtained radiation image is degraded.
In the mean time, Japanese Unexamined Patent publication No. 7(1995)-027864 discloses a radiation detector in which sharpness degradation is prevented by changing the arrangement order of the scintillator and solid state photodetector, i.e., the solid state photodetector is disposed in front of the scintillator viewed from the direction from which the radiation is irradiated.
Further, Japanese Unexamined Patent Publication No. 7(1995)-027865 proposes a radiation detector in which a scintillator is provided on each side of the solid state photodetector, and irradiated radiation is converted to light by the scintillators to improve the detective quantum efficiency (DQE) compared to the conventional radiation detector having a scintillator only on one side.
The radiation detector proposed in Japanese Unexamined Patent Publication No. 7(1995)-027865, however, includes a substrate for supporting the solid state photodetector, which has a thickness greater than several hundred micrometers to ensure sufficient strength. Further, the rear light receiving surface of the solid state photodetector viewed from the direction from which the radiation is irradiated is arranged such that it invariably receives fluorescence transmitted through the substrate, so that the fluorescence is dispersed while transmitting through the substrate, causing a problem of degraded image sharpness.
Still further, Japanese Unexamined Patent Publication No. 9(1997)-145895 proposes a radiation detector that has solved the aforementioned problem by using a fluorescent glass substrate, as the scintillator disposed behind the solid state photodetector viewed from the direction from which the radiation is irradiated, on which the solid state photodetector is integrally formed.
In the radiation detector proposed in Japanese Unexamined Patent Publication No. 9(1997)-195895, however, the fluorescent glass substrate disposed behind the solid state photodetector viewed from the direction from which the radiation is irradiated also has high transparency to light propagating in the direction parallel to the surface of the fluorescent glass substrate. Thus, the visible light converted by the fluorescent glass substrate is spread within the substrate, causing a problem of degraded sharpness of obtained radiation images.
In view of the circumstances described above, it is an object of the present invention to provide a radiation detector capable of improving the sharpness of radiation images.